1. Field of the Invention
The present invention generally relates to an apparatus for dropping plugs into a wellbore. More particularly, the invention relates to a plug-dropping container for releasing plugs and other objects into a wellbore, such as during cementing operations.
2. Description of the Related Art
In the drilling of oil and gas wells, a wellbore is formed using a drill bit that is urged downwardly at a lower end of a drill string. After drilling a predetermined depth, the drill string and bit are removed and the wellbore is lined with a string of casing. An annular area is thus formed between the string of casing and the formation. A cementing operation is then conducted in order to fill the annular area with cement. The combination of cement and casing strengthens the wellbore and facilitates the isolation of certain areas of the formation behind the casing for the production of hydrocarbons.
It is common to employ more than one string of casing in a wellbore. In this respect, a first string of casing is set in the wellbore when the well is drilled to a first designated depth. The first string of casing is hung from the surface, and then cement is circulated into the annulus behind the casing. The well is then drilled to a second designated depth, and a second string of casing, or liner, is run into the well. The second string is set at a depth such that the upper portion of the second string of casing overlaps the lower portion of the first string of casing. The second liner string is then fixed or xe2x80x9chungxe2x80x9d off of the existing casing. Afterwards, the second casing string is also cemented. This process is typically repeated with additional liner strings until the well has been drilled to total depth. In this manner, wells are typically formed with two or more strings of casing of an ever-decreasing diameter.
In the process of forming a wellbore, it is sometimes desirable to utilize various plugs. Plugs typically define an elongated elastomeric body used to separate fluids pumped into a wellbore. Plugs are commonly used, for example, during the cementing operations for a liner.
The process of cementing a liner into a wellbore typically involves the use of liner wiper plugs and drill-pipe darts. A liner wiper plug is typically located inside the top of a liner, and is lowered into the wellbore with the liner at the bottom of a working string. The liner wiper plug has radial wipers to contact and wipe the inside of the liner as the plug travels down the liner. The liner wiper plug has a cylindrical bore through it to allow passage of fluids.
After a sufficient volume of circulating fluid or cement has been placed into the wellbore, a drill pipe dart or pump-down plug, is deployed. Using drilling mud, cement, or other displacement fluid, the dart is pumped into the working string. As the dart travels downhole, it seats against the liner wiper plug, closing off the internal bore through the liner wiper plug. Hydraulic pressure above the dart forces the dart and the wiper plug to dislodge from the bottom of the working string and to be pumped down the liner together. This forces the circulating fluid or cement that is ahead of the wiper plug and dart to travel down the liner and out into the liner annulus.
Typically, darts used during a cementing operation are held at the surface by plug-dropping containers. The plug-dropping container is incorporated into the cementing head above the wellbore. Fluid is directed to bypass the plug within the container until it is ready for release, at which time the fluid is directed to flow behind the plug and force it downhole. Existing plug-dropping containers, such as cementing heads, utilize a variety of designs for allowing fluid to bypass the plug before it is released. One design used is an externally plumbed bypass connected to the bore body of the container. The external bypass directs the fluid to enter the bore at a point below the plug position. When the plug is ready for release, an external valve is actuated to direct the fluid to enter the bore at a point above the plug, thereby releasing the plug into the wellbore.
Another commonly used design is an internal bypass system having a second bore in the main body of the cementing head. In this design, fluid is directed to flow into the bypass until a plug is ready for release. Thereafter, an internal valve is actuated and the flow is directed on to the plug.
There are disadvantages to both the external and internal bypass plug container systems. Externally plumbed bypasses are bulky because of the external manifold used for directing fluid. Because it is often necessary to rotate or reciprocate the plug container, or cementing head, during operation, it is desirable to maintain a compact plug container without unnecessary projections extending from the bore body. As for the internal bypass, an internal bypass requires costly machining and an internal valve to direct fluid flow. Additionally, the internal valve is subject to erosion by cement and drilling fluid.
In another prior art arrangement, a canister containing a plug is placed inside the bore of the plug container. The canister initially sits on a plunger. Fluid is allowed to bypass the canister and plunger until the plug is ready for release. Upon release from the plunger, the canister is forced downward by gravity and/or fluid flow and lands on a seat. The seat is designed to stop the fluid from flowing around the canister and to redirect the flow in to the canister in order to release the plug. However, this design does not utilize a positive release mechanism wherein the plug is released directly. If the cement and debris is not cleaned out of the bore, downward movement of the canister is impeded. This, in turn, will prevent the canister from landing on the seat so as to close off the bypass. If the bypass is not closed off, the fluid is not redirected through the canister to force the plug into the wellbore. As a result, the plug is retained in the canister even though the canister is xe2x80x9creleased.xe2x80x9d
The release mechanism in some of the container designs described above involves a threaded plunger that extends out from the bore body of the container, and requires many turns to release the plug. The plunger adds to the bulkiness of the container and increases the possibility of damage to the head member of the plug container. Furthermore, cross-holes are machined in the main body for plunger attachment. Because a plug container typically carries a heavy load due to the large amount of tubular joints hanging below it, it is desirable to minimize the size of the cross-holes because of their adverse effect on the tensile strength of the container.
Therefore, there is a need for a more effective plug-dropping apparatus for a cementing head. There is a further need for a cementing head that can efficiently release a plug into a wellbore. There is still a further need for a plug releasing apparatus that is more compact, easier to handle, and less expensive to manufacture.
The present invention generally relates to a plug-dropping container for use in a wellbore circulating system. An example of such a system is a cementing operation for a liner string. The plug-dropping container first comprises a tubular housing having a top end and a bottom end. The top end is in sealed fluid communication with a wellbore fluid circulation device. Thus, fluid injected into the cementing head will travel through the housing before being injected into the wellbore.
The plug-dropping container also comprises a canister disposed co-axially within the housing. An annulus is thus defined between the canister and the surrounding housing. The canister is likewise tubular in shape so as to provide a fluid channel therein. The canister also has a top opening and a bottom opening. However, the canister is configured so that it is movable axially within the housing. A bypass gap is left between the top opening of the canister and the bore of the head member. In one aspect of the invention, the bypass gap is created by configuring the length of the canister to be less than the length of the surrounding housing.
The canister is axially movable within the housing. In this respect, the canister can be moved axially within the housing from a lower position to an upper position. In its lower position, fluid is permitted to flow from the bore of the head member, through the bypass gap, and into the annular area around the canister. Fluid may thus bypass the channel within the canister. However, raising the canister to its upper position within the housing causes the top opening to approach the bore of the cementing head. This effectively shuts off the bypass gap, thereby forcing fluid to be injected into the wellbore through the canister channel.
The plug-dropping container is used to retain one or more plugs such as a drill pipe dart for a cementing operation. In this respect, the channel of the canister is configured to closely receive the dart. While the dart is retained within the canister, the canister is in its lower position. This permits fluid to travel around the canister and the dart therein. When the dart is to be dropped into the wellbore, the canister is raised so as to substantially shut off fluid flow through the bypass gap. This forces fluid to flow into the channel of the canister. Fluid pressure builds behind the dart, forcing it out of the canister.
The plug-dropping container finally comprises a plug-retaining device. In one aspect, the plug-retaining device is a tubular member having a fluid channel therein. The plug-retaining device also has a first end, a second end, and a wall therebetween. When the plug-dropping container is in its plug-retained position, the plug-retaining device is oriented such that the wall of the plug-retaining device blocks the downward flow of the dart. In this position, the dart prohibits the flow of fluid through the canister; instead, fluid travels around the canister and through the canister annulus.
At the point at which plug-release is desired, the canister is raised within the housing. In one aspect of the assembly of the present invention, this is accomplished by rotating the plug-retaining device. The plug-retaining device is rotatable between a plug-retained position and a plug-released position. In the plug-retained position, the plug-retaining device is turned such that it blocks the canister channel and prevents dropping of the plug. Blocking the canister channel causes fluid entering the housing to flow around the canister via the bypass gap. To release the plug, the plug-retaining device is rotated by turning one or more shafts connected thereto. Rotation of the shaft causes the canister to move up axially and to approach the bore of the head member, thereby closing off the bypass gap and directing fluid to flow directly into the channel of the canister. Turning the plug-retaining device to the plug-released position also causes the plug-retaining device channel to be in fluid communication with the canister channel. The plug-retaining device channel can then receive the plug, whereupon the plug is released into the wellbore. The plug-retaining device is then in position to receive both the dart and fluid flowing through the cementing head.
In another embodiment, one or more plug-dropping containers of the present invention may be stacked for sequential release of more than one plug in a cementing operation.